Jet Compressors, also called Thermo-Compressors, are commonly used to provide mixture of air and a high quality gaseous fuel such as natural gas to supplement the volume of a lower quality gaseous fuel such as coke-oven-gas. In these devices a jet of the motive gas directed from a nozzle into the throat of a venturi section, called the diffuser, produces suction which draws air or other gas into the diffuser thereby diluting the motive gas. The mixed gas thus produced is discharged from the exit end of the diffuser into a mixed gas manifold whence it flows into the gas distribution system and eventually to the users. The mixture provided should match a combustion index, such as the Wobbe Index (Calorific Valve/.sqroot.Specific Gravity) or the Air Requirement Index (Stoichiometric Air/.sqroot.Specific Gravity) of the lower quality gas, such as coke-oven-gas, being supplemented.
The manufacturers of jet compressors adjust nozzle size, diffuser throat size, and nozzle to throat distance to provide a mixture of the desired composition as long as the motive force, mixed gas manifold pressure (also called the discharge pressure), and the pressure drop in the air intake are maintained within rather narrow limits. The composition of the mixture is very sensitive to the discharge pressure. If the mixed gas manifold pressure is allowed to rise above the design discharge pressure, the suction pressure is reduced, less air is entrained, and the mixed gas gets rich. Conversely, if the manifold pressure is allowed to fall very much below the design discharge pressure the suction increases, more air is entrained, and the mixed gas gets leaner. Either of these conditions can cause the user to waste fuel.
In practical operation rapid changes in usage, supply of the source gas, or other such factors are common, which results in fuel wastage.